1. Field of the Invention
The present invention relates to an optical wavelength converter device for converting a wave having a fundamental frequency, hereinafter the fundamental, to its second harmonic, which has a wavelength which is 1/2 the wavelength of the fundamental, and more particularly to an optical wavelength converter device of the fiber type for eliminating phase interference between waves traveling in a guided mode in which a fundamental is guided through a core and waves traveling in a radiation mode in which a second harmonic is radiated into a cladding.
2. Description of the Prior Art
Various attempts have heretofore been made for converting the wavelength of a laser beam into a shorter wavelength based on the generation of a second harmonic by a nonlinear optical material. One example of an optical wavelength converter device for effecting such laser wavelength conversion is a bulk crystal type converter device as disclosed, for example, in Introduction to Optical Electronics written by A. Yariv and translated by Kunio Tada and Takeshi Kamiya (published by Maruzen K. K.), pages 200-204. This optical wavelength converter device relies upon the birefringence of a crystal in order to meet phase matching conditions. Therefore, any material which does not exhibit birefringence or exhibits only small birefringence cannot be employed even if it has high nonlinearity.
A fiber type optical wavelength converter device has been proposed to solve the above problem. The optical wavelength converter device of this type is in the form of an optical fiber comprising a core made of a nonlinear optical material surrounded by a cladding. One example of such an optical fiber is shown in Vol. 3, No. 2, pp. 28-32 of the bulletin of the microoptics research group of a gathering of the Applied Physics Society. Recently, many efforts have been directed to the study of fiber type optical wavelength converter devices since they can easily eliminate interference between waves traveling in a guided mode in which a fundamental is guided through the core and waves traveling in a radiation mode in which a second harmonic is radiated into the cladding (for so-called Cherenkov radiation).
One problem which has been recognized with respect to such a fiber type optical wavelength converter device is that the phase of the second harmonic tends to be disturbed while the second harmonic is traveling toward an end of the device by being repeatedly and totally reflected between outer surfaces of the cladding. When the phase of second harmonic is disturbed, the intensity of the second harmonic which is eventually emitted from the optical wavelength converter device is lowered and so is the wavelength conversion efficiency. It also becomes impossible to sufficiently converge the second harmonic that has been emitted from the converter device.
Moreover, the second harmonic is apt to be absorbed by the core as it travels through the fiber type optical wavelength converter device while repeatedly undergoing total reflection. Such second harmonic absorption also lowers the wavelength conversion efficiency, and makes it difficult to extract a second harmonic with a high intensity, which is a short and easily absorbed wave.